Adding value or not?

[Robert Fabian]

The house is a 4 bedroom, 1 1/2 bath colonial. The four bedrooms are all on the upper level of the house and they all share one bathroom. Would it add value or take away value if I turned a smaller bedroom adjacent to the master bedroom into a master bath and a walk in closet. It has neither as of now. This would make my rehab now into a 3 bedroom 2 1/2 bath. Any advice?